1. 1.

    Velocity thresholds of neurones in the displacement-sensitive pathway of the crayfish were investigated using a stimulus programme of ramp fluid movements, with a range of velocities from 0.01-55 cm s−1 at the source (in both forward and backward directions).

  2. 2.

    The velocity of the stimulus was calculated to produce particle movements with velocities of between 0.001-1.0cms−1 at the receptors of the telson.

  3. 3.

    The distribution of threshold values for afferents in the fourth root of the sixth abdominal ganglion and interneurones of abdominal 4–5 connectives was plotted. Thresholds were similar for afferents and interneurones, differing in absolute value from 0.001-1.0cms−1. Although neurones had threshold values throughout this range, there were peaks in the distributions. For the sample of 143 interneurones these were at 0.002-0.004, 0.01-0.08, 0.1-0.4cms−1. In the sample of 43 afferents the middle peak fell at 0.02-0.06cms−1, but the other two were the same as those for the interneurones.

  4. 4.

    The threshold for a behaviour pattern which makes crayfish turn to face the water current was around 0.1 cms−1 continuous fluid speed.

  5. 5.

    Escape behaviour elicited by pure displacement stimuli was observed in our experiments only in animals of 2–3 cm body length which responded to displacement amplitudes of 28 mm s−1 and 6.7 cms−1 maximum particle velocity.

  6. 6.

    Strongly phasic interneurones in the pathway obviously contribute significantly in triggering escape and need high acceleration transients to become fully active.

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